This invention relates to alkyl esters of polyene macrolide antibiotics and to convenient and safe synthesis thereof by methods suitable for large scale production.
U.S. Pat. No. 3,945,993 to Schaffner et al. relates to the preparation of esters of Amphotericin B and other amphoteric polyene macrolides by dissolving the polyene macrolide in methyl sulfoxide and diluting with tetrahydrofuran. The resulting suspension is treated with diazomethane to produce the methyl ester. The ester product is then recovered from the reaction mixture by precipitation with ethyl ether.
U.S. Pat. No. 4,035,568, also to Schaffner et al., relates to processes for making Amphotericin B derivatives using diazomethane or diazapropane.
Belgian Patent No. 802,512 relates to another method for synthesis of the methyl ester of Amphotericin B. Aqueous ammonium hydroxide is added to a solution of Amphotericin B in methyl sulfoxide to approximately pH 10. Treatment of this mixture with diazomethane yields the methyl ester.
U.S. Pat. No. 4,035,567 describes another procedure for the preparation of Amphotericin B methyl ester. Amphotericin B is mixed with either dimethylformamide (DMF) or hexamethylphosphoric triamide (HMPA) for a specific time period. Ammonium hydroxide is then added to raise the pH of the mixture above 9 with DMF or above 10 with HMPA. Diazomethane is then added in each case to form the methyl ester.
The use of diazomethane with unprotected Amphotericin B or other amphoteric polyene macrolides to produce the corresponding methyl esters is most often associated with the formation of undesired by-products. These often consist of compounds wherein methylation has occurred at sites in the molecule in addition to the carboxyl function.
U.S. Pat. No. 4,235,993 describes an attempt to limit the formation of these polymethylated Amphotericin B derivatives. Included are several synthetic steps during which the intermediates, namely N-benzylidine Amphotericin B and its methyl ester, are formed. Amphotericin B is dissolved in equal parts of DMF and dimethyl-sulfoxide (DMSO), and benzaldehyde is then added to form the Schiff base, N-benzylidine Amphotericin B. Treatment of this intermediate with diazomethane in tetrahydrofuran (THF) yields the intermediate product, N-benzylidine Amphotericin B methyl ester. Treatment of this intermediate with acids in aqueous medium hydrolyzes the Schiff base and produces the water-soluble Amphotericin B methyl ester salt.
Most of the available procedures to protect an amine block the amino function as an amide or carbamate (see T. W. Greene, "Protecting Groups in Organic Synthesis", J. Wiley Publ., New York, 1981). The FMOC (fluorenylmethylcarbonyl) group has been employed to protect the amino group of Amphotericin B (M. J. Driver et. al., Tetrahedron Letters, 33, 4357-4360 (1992). Diazomethane or methyl iodide with diisopropylethylamine is then utilized to esterify the carboxyl group and the FMOC protecting group is removed in a second reaction. It is unexpected that use of the Schiff base to protect the amino group is possible in the esterification with an alkylating agent since it might be expected that an alkylating agent such as methyl iodide or dimethyl sulfate would alkylate the imino nitrogen forming an N-methyl compound. The major additional advantages of using the Schiff base protecting group are solubilizing of the Amphotericin B by disruption of its amphoteric character and ease of removal of the protecting group by contact with water under relatively mild conditions which do not effect the sensitive groups in the polyene molecule. An additional benefit is the steric interaction of the Schiff base which prevents alkylation of the hydroxyl groups on the glycosamine function.
Amphotericin B is a polyene macrolide antibiotic useful in a variety of applications including human treatment of systemic fungal diseases and agricultural treatment of fungal plant infections. Amphotericin B and its methyl ester have also been shown to have a broad spectrum of antiviral and anticancer activity. The structure of Amphotericin B (AMB) and its methyl ester (AME) are illustrated below: ##STR1## AMPHOTERICIN B: R.dbd.H AMPHOTERICIN B METHYL ESTER: R.dbd.CH.sub.3